Due to relatively high instances of system inertia and delay in automotive transmissions, using exclusively feedback control of various components in automotive transmissions may not be sufficient for certain transient maneuvers, especially for systems with large accumulators. Control of transmission turbine speed during a kickdown shift is one example of a transient condition in automotive transmissions. During a kickdown shift, such as a drop from 4th gear to 3rd gear, or from 3rd gear to 2nd gear, the speed of the turbine must increase to correspond to a targeted gear ratio. Accordingly, the acceleration of the turbine must be controlled to correspond to a targeted acceleration based on current gear and vehicle speed acceleration. In such transient cases, feedforward control may be used to anticipate system changes. For example, mixed feedforward and feedback control can be used for a smooth kickdown shift without causing significant “feel” issues for the driver, thereby improving overall shift quality. Shift quality has been shown to be an important factor for driver satisfaction.
During kickdown shifts, the engine output speed increases, thereby increasing torque converter slip and torque output. Vehicle speed and throttle position trigger a downshift schedule, and a kickdown shift is initiated. One current transmission control method 10 effects a kickdown shift by dumping clutch element pressure until slip occurs as shown in FIG. 1. The engine output speed increases, thereby increasing torque converter slip and torque output, while release element pressure continues to fall. Therefore, when slip occurs, a release element controlled speed phase 12 begins, and proportional control is used on the release element to limit the rate of turbine speed increase, or turbine acceleration 14.
The input torque is primarily used to accelerate the engine, the torque converter, and the turbine during the release element controlled speed phase 12. After the release element controlled speed phase 12, the targeted turbine acceleration 14 is reduced by a step change, and the speed of the engine is decreased in such a manner that when Nt, or current turbine speed 16, passes through Nj, or target gear speed 17, the resulting overspeed and/or runaway is minimized. The phase 18 in which this activity occurs is referred to as “feather control.” After the feather control phase 18 is complete, the control method 10 attempts to limit the current turbine speed 16 to a relatively small value, such as 50 rpm, above the target gear speed 17 during a second release element controlled speed phase 19, and the turbine acceleration 14 continues to decrease incrementally. Additionally, an apply element fill event is timed to occur a short time before the current turbine speed 16 reaches the targeted gear speed 17. For example, the apply element fill event may be timed to occur 100 ms before the current turbine speed 16 reaches the targeted gear speed 17. The apply clutch will be filled to a maximum pressure when the turbine speed 16 reaches within a trigger limit of the targeted gear speed 17.
However, the transmission control method 10 may not function as described above in actual practice. For example, the current turbine speed 16 may overshoot the target gear speed 17, or exceed the control capabilities of the transmission control method 10, during a kickdown shift. Because the turbine and associated planetary gear sets have inertia, and the hydraulic control system experiences system delay, any significant change in desired acceleration may cause temporary loss of control and/or unstable control dynamics. Additionally, turbine speed changes between gears can be substantially different at different gears or different vehicle speeds. Therefore, it is desirable to provide optimized control during a kickdown shift to further improve shift quality. A continuous variable and speed-based desired acceleration method to provide consistent and accurate transmission control during a kickdown shift is proposed.